4. Sounds obvious, but check the 5 day weather forecast which is now much more reliable and gives you a good idea what to expect whilst you are away. If no rain is forecast for the first few days water heavily before you leave for holiday. Water effectively especially new plants and shrubs. It is the roots which need the water (image right) not the foliage (image left) either puddle water around the plant to ensure the roots get a good soaking, or sink a bottle alongside the plant so you can water directly to the roots.
5. Equally if a lot of rain is forecast, make sure none of your containers or plants are placed in trays as they will quickly fill up with rainwater. Plants dislike standing in water for prolonged periods.
6. Capillary matting can be useful and will nurse a plant through a week (as long as it's not a heat wave week) Soak the matting and set the plant on it to keep hydrated.
7. If you have a lot of sensitive plants such as tomatoes and veg in containers and baskets, a drip feed system connected to the mains with a timer is ideal, (but only if there is not a hosepipe ban.)
8. If this is not possible move all containers into a shady spot, water well and mulch. Simply putting containers into shade makes a great deal of difference in terms of water loss. Give everything a really good soak before you go, group containers together in shady areas and mulch surfaces